That's the thing, those plates are really for aesthetics, not really ergonomics. They also do nothing to alter the tone. As it is now I anchor my thumb on the pickup, I'd imagine I might anchor my thumb on pickup cover if I installed it. Like I said, I want to play one for a while with covers already installed before I actually take the drill to it.
IIRC the story goes that the so called "thumb rest" was an original design flaw from Leo Fender, as the electric bass hadn't been really "invented" yet he imagined bass players would use their thumbs to play the instrument and put a rest to anchor the other fingers....obviously this wasn't the case for MOST bassists, although I have seen some bass players in pro bands use their thumbs. I think people just like them there because it "looks classic"
What I think I might do with the thumbrest considering this pickguard didn't come with the holes predrilled is this:
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